What is the greatest common factor of $40$ and $36$ ? Another way to say this is: $\operatorname{gcf}(40, 36) = {?}$
Solution: The greatest common factor is the largest number that is a factor of both $40$ and $36$ The factors of $40$ are $1$ $2$ $4$ $5$ $8$ $10$ $20$ , and $40$ The factors of $36$ are $1$ $2$ $3$ $4$ $6$ $9$ $12$ $18$ , and $36$ Thus, the greatest common factor of $40$ and $36$ is $4$. $\operatorname{gcf}(40, 36) = 4$